1. Field of the Invention
The present invention relates to an image forming apparatus having a pixel clock generator for generating a pixel clock to switch, on a pixel unit basis, an image signal which is sent to a laser beam modulator and to an image forming method which is executed in such an apparatus.
2. Description of the Related Art
In an electrophotographic system for exposing an image by a laser beam, a method whereby the laser beam is irradiated to a rotary polygon mirror (polygon mirror) and deflected and the surface of a photosensitive material is exposed by the deflected reflection light has been used. As a photosensitive material, it is desirable to use a photosensitive material having a shape which is away from a laser beam light source at an equal distance, that is, an arc shape from a reflecting surface of the polygon mirror. However, in many image forming apparatuses, a cylindrical photosensitive material is used in order to form an image after exposure.
By using the cylindrical photosensitive material, lengths of optical paths from the respective light sources to the photosensitive material differ and a difference occurs in a main scanning magnification by the laser beam. As a method of correcting the difference of the main scanning magnification, there is a construction in which an optical device such as an fθ lens or the like is provided between the light source and the photosensitive material. However, high precision is required for the correction by the optical device. Therefore, in many image forming apparatuses, the high-precision correction of the optical device is not performed but a main scanning length is divided into a plurality of areas and the magnification in the main scanning direction is adjusted by inserting or extracting a micro pixel (hereinbelow, referred to as a “pixel segment”) on a division area unit basis.
A clock frequency showing the pixel segment is obtained by a method whereby a clock to transfer image data of one pixel is multiplied by a value corresponding to desired resolution. That is, assuming that the resolution is equal to 16, the clock frequency showing the pixel segment is set to a frequency which is 16 times as high as the clock frequency for transferring one pixel. If the resolution is equal to 32, the clock frequency is set to a frequency which is 32 times as high as the clock frequency for transferring one pixel.
In many cases, each of the pixel clock frequency and the clock frequency of the pixel segment differs every print unit (engine) of the electrophotographic system. That is, each of the pixel clock frequency and the pixel segment clock frequency differs every image forming apparatus of the electrophotographic system. Japanese Patent Application Laid-Open No. 2004-237623 discloses such a mechanism that a reference clock frequency is set from the pixel clock frequency of the print unit of the electrophotographic system and the reference clock is multiplied to the clock frequency of a high frequency by a PLL (Phase Locked Loop). It is also disclosed that the apparatus uses a construction in which on the basis of a multiplication clock serving as a clock frequency of the pixel segment, three kinds of pixel clocks for controlling a micro zoom ratio in the main scanning direction are formed and selectively output to the print unit of the electrophotographic system. The three kinds of pixel clocks are a reference clock frequency to transfer one pixel, a clock frequency in which a unit time of the reference clock frequency is extended by a time corresponding to one pixel segment, and a clock frequency in which the unit time of the reference clock frequency is shortened by the time corresponding to one pixel segment. It is also disclosed that in the magnification control of every area of the main scanning length, a plurality of selection signals of time-sequential distribution for selecting the three kinds of pixel clocks are held and by synthesizing the time-sequential distribution selection signals, a selection output signal of the pixel clocks is formed.
However, in a pixel clock generator or the image forming apparatus according to Japanese Patent Application Laid-Open No. 2004-237623, as selection signals which are used to select the pixel clocks which are output to the print unit of the electrophotographic system, it is necessary to hold the selection signals of the number as many as the pixels in the main scanning direction. A construction of a memory for holding the selection signals or a holding unit corresponding to such a memory causes a circuit scale to be increased and becomes a factor of an increase in costs. Since the three kinds of pixel clocks are always formed, there is a possibility of an increase in radiation noise level of the image forming apparatus. Further, since the frequency of the reference clock which is used to form the pixel clock conforms with the print unit of the electrophotographic system constructing the image forming apparatus, there is a possibility that it differs from the clock frequency to control an image processing unit for processing the image data and serving as a transferring source. In such a case, at least two kinds of oscillators are necessary to construct the image forming apparatus and it becomes a factor of an increase in costs.